This invention relates to pressure suppression apparatus of a nuclear power plant, and more particularly, pressure suppression apparatus which can operate safely a nuclear reactor by suppressing the pressure rise in a reactor containment and absorbing thermal energy uniformly in a case when a coolant is released in the reactor containment.
In a nuclear power plant, when an accident, such as loss of coolant caused by breakage of a primary cooling system of a nuclear reactor, occurs or when a safety valve is required to be opened for releasing main steam in a pressure vessel, it is required for the pressure suppression apparatus to promptly absorb, within a primary reactor containment, the thermal energy of the steam discharged from steam exhaust pipes of safety valves and exhaust pipes of a part of turbines for operating pumps of a reactor cooling system for emergency, to suppress the pressure rise in the reactor containment and to prevent the escape of steam or water in the reactor to the outside thereof. The pressure suppression apparatus further serves as a water source for the reactor cooling system in the form of a closed loop for emergency use. Thus, the pressure suppression apparatus operates for maintaining the safe operation of the nuclear power plant.
Although various types of reactor containment are different in their structures, description is made hereunder as to Mark II type reactor containment. However, the invention is also applicable to the other types of reactor containment.
The inner space of a reactor containment surrounding a reactor pressure vessel is divided by a diaphragm floor into two parts, the upper one serving as a drywell space while the lower one as a suppression chamber. A water pool is contained in the suppression chamber. A plurality of vent pipes are suspended from the diaphragm floor and one end of each vent pipe is opened in the drywell and the other end is opened in the water pool in the suppression chamber. A plurality of main steam pipes are connected to the reactor pressure vessel to feed steam to the turbine, and safety valves for releasing the main steam are provided for the respective main steam exhaust pipes. The exhaust pipes of the safety valves extend water-tightly through the diaphragm floor and are opened in the water pool of the suppression chamber. The exhaust pipe of a pump-operating turbine of an emergency reactor cooling system extends through the reactor containment and is also opened in the water pool.
It is known that a number of small openings are provided to the ends of the steam exhaust pipes immersed in the water pool and these openings are provided for the purpose that when a coolant such as steam is discharged from the pressure vessel into the water pool through vent pipes, pipes for discharging steam discharged from safety valves for releasing main steam, or exhaust pipes of the turbines, the thermal energy of the steam can be promptly absorbed by the water pool and uniformly condensed therein.
However, it has been desired to construct safely and effectively the pressure suppression apparatus by taking the following points into consideration.
(1) Usually, the exhaust pipes provided with discharge openings immersed in water pool for discharging steam are initially filled with non-condensible gas such as nitrogen gas or air. For this reason, in the case where the coolant is rapidly released into the exhaust pipes, the non-condensible gas under super-compressed condition is discharged into the water pool in the suppression chamber, and non-condensible gas layer is formed near the exhaust pipes thereby forcing upwardly the water pool. Such phenomenon is not desired for the structure of the suppression chamber. PA1 (2) Since uniform condensation of the coolant such as steam discharged from the exhaust pipes into the water pool may induce vibration of the pipes, the coolant should be condensed uniformly in the water pool. PA1 (3) The non-condensible gas under super-compressed condition may cause unstable oscillation, i.e., super-expansion of the gas, which would damage the wall of the suppression chamber.